In the fabrication of composite structures, plies are made and thereafter joined or bonded together to make a final composite product. A first example method of bonding plies into a composite product may include pre-impregnating plies of various fiber forms (e.g., fiber tows, unidirectional tape, woven fabric, braided fibers, etc.) with a resin mixture and then laying up these prepreg composite plies to form a composite layup. A second example method of bonding plies into a composite may include laying up dry fiber plies/preforms and then infusing the at least part of the ply stack/preform with resin. In both the first and second examples, individual plies may be oriented at different angles such as 0°, ±20°, ±30°, ±45°, ±60° or 35 90° to control the strength, modulus and other mechanical properties of the composite in multiple directions. Current plies used in the fabrication of composite products are typically, approximately 0.0075 inches in thickness per ply. At times, a distinct layer of toughener material is positioned to overlie the outside of the ply to enhance the performance of that ply.
It has been seen that benefits to microcrack resistance and mechanical performance occur when thinner plies are used. However, these benefits come with a significant cost particularly with respect to the time to manufacture. Thinner plies are constructed using standard layup processes. However, if, for example, the thinner ply to be fabricated is one half the thickness of the typical ply, twice the linear amount of plies would need to be produced to achieve the final thickness, which results in double the manufacturing laydown time and cost.
In the present process for fabricating composite materials, plies are made by spreading fiber tows to a width that will provide the desired final ply thickness. These spread fiber tows are arranged side by side during the manufacturing process resulting in a uniform fiber bed wherein the separate fiber tows are no longer distinct. With a uniform fiber bed being created, it is difficult to provide through thickness or z-direction performance. Z-direction performance has been enhanced in the past by adding through thickness stitching, z-pins and adding toughener materials between plies. These methods tend to increase cost in manufacturing and the complexity of material, as well as, arming other performance attributes such as increasing micro-cracking propensity, reducing open hole compression, etc.
There is also a need for in-plane performance enhancement. Enhancing in-plane performance has been accomplished by using plies having fibers positioned or oriented in different angular positions relative to other plies in that composite material construction. Additionally, in-plane enhanced performance has been addressed by adding materials such as thermoplastic tougheners, veils, filaments, films and particles between plies when assembling the composite material product.
There is a need to stabilize fibers within a tow when assembling the ply. UD tapes have been previously stabilized by controlling the viscosity and tack of the resin as well as through the use of binders/2D veil combinations in dry fiber formats.